For horses, the long, hot summer often means days spent in the sun, heat on your horse’s coat and seeking out shade to cool down. For most horses, sweating is a natural reaction that allows the horse’s core body temperature to remain at the proper temperature. If the horse’s core temperature rises excessively, he may overheat, resulting in muscle tremors, heat stress, and impaired organ function and in some cases, collapse. But what if the horse cannot sweat? Anhidrosis (or “dry coat”, where affected horses are sometimes referred to as “puffers) is a condition where horses are unable to sweat and as such, are unable to self-cool, which is a potentially major problem for owners during periods of hot weather.
Anhidrosis appears to only be found in areas of the world where the weather is hot and humid. Between 5-25% of horses in these locations suffer from this condition where they are unable to sweat. It is believed to be caused by decreased or absent response to sweat gland stimulation by epinephrine (the hormone and neurotransmitter that triggers normal sweat gland activity.) The reason for this compromised response is, however, unknown. There appears to be a breed predisposition to developing the condition in Thoroughbreds, Quarter Horses, and Warmbloods but it is rarely seen in Arabians. It is thought that there are complex factors that determine whether an Anhidrotic horse’s offspring will develop the same problem.
It is important to keep your eye out for signs of Anhidrosis in hot weather, it can develop at any time and may be more visible in horses who are regularly exercised. Signs can include:
During exercise, a significant proportion of the heat generated in the horse’s body will be lost via sweat evaporation with another 25% lost via the respiratory system. When the horse cannot sweat, this puts great stress on the respiratory system to help cool the body and as such, horses with Anhidrosis will display higher respiratory rates, even to the extent of puffing and panting fast and loudly (hence the name, Puffers!) for prolonged periods after exercise. This symptom occurs both during exercise and when horses are at rest during warm and/or humid weather.
Diagnosis is normally predicated on a combination of the horse’s history and the veterinarian’s observation of clinical signs. If a small amount of epinephrine is injected into the intradermal layer (between the skin layers), and there is an inhibited sweat response, this can be seen as a definitive diagnosis. At present there is no specific treatment and there is no known cure. Careful management based on your veterinarian’s input is the recommended strategy, when dealing with Anhidrosis.
If it is an option, you may wish to consider selling your horse or leasing to an owner who resides in a cooler climate. There is a possibility that the change in weather conditions will prompt him to start sweating again. If you prefer to keep your horse and not rehome him, there are some management practices which can help.
Anhidrosis is a tricky condition to manage, but with careful planning you can help your horse enjoy the hot weather without overheating.
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